The present invention relates generally to medical electrodes of the type adapted to be attached to the skin, and more particularly to improved medical skin electrodes for use in both measuring biological or physiological potentials and transmitting stimulating currents.
As is well known, biological or physiological potentials can be sensed at the skin of a living body by using a pair of electrodes spaced apart at select locations on the body (such locations being dependent on the type of potentials being measured), and can be recorded by appropriate instrumentation. For example, these electrodes can be used to monitor and record cardiopotentials from the chest and limbs, electrooculographic signals indicating eye motions by applications to appropriate positions on the face, transthoracic impedance differentials indicating apneic spells, as well as employed for long term testing and monitoring such as Holter cardiac arythmia stress testing.
In recent years, various techniques have been developed in promoting healing within the body by using a stimulating current and which utilize one or more medical electrodes attached to the skin. For example, one such technique can be used with rather a high rate of success in promoting healing of fracture nonunions. One instrumentation system which has been developed to promote osteogenesis, where bone fragments otherwise fail to join together, is commercially-available from Zimmer, USA of Warsaw Ind. under the name "Direct Current Bone Growth Stimulator". The stimulator provides a constant direct current directly at the site of a fracture nonunion through percutaneously placed cathodes. The cathodes are in the form of a plurality of pins whose ends are inserted percutaneously and typically disposed in the nonunion site. The anode of the system is in the form of an electrode pad suitably disposed on the skin in a selected location so that when a constant direct current power supply is properly connected to the cathode pins and anode pad the appropriate approximate current level, typically at about 80 microamps, is generated in the region of the nonunion.
Development of such direct current stimulation devices has created problems associated with available electrodes for use as anode pads. More specifically, presently available standard electrodes typically include a stud member for connection to an electrical lead. The stud member usually extends through a sheet of adhesive to a pad of porous material preloaded with an electrically-conductive gel so that the electrode can be attached to the skin maintaining the gel in contact with the skin while at the same time exposing the stud member so that it can be connected to an electrical lead. The stud member of at least some commercially available electrodes typically is made of an electrically non-conductive material, such as a polymeric plastic, to form a core. The plastic core is then coated with a layer of silver, which provides a good electrically-conductive path between the electrical lead and the gel material with the interface between the silver and gel providing a sufficiently low impedance path.
The gel material typically contains an agar solution of dissolved sodium chloride so that the gel is completely compatible with the skin and generates a minimum of motion artifacts when in operation.
The chemistry of such an electrode stud member and gel is therefore such that an ionization reaction occurs when a current flows through the electrode and gel. Specifically, the dissolved salt provides Na.sup.+ and Cl.sup.- ions in the gel solution. When applying an AC signal, the silver oxidizes and reacts with the chloride to form silver chloride (with silver atoms losing electrons to form Ag.sup.+ ions and the Ag.sup.+ ions and Cl.sup.- ions combining to form silver chloride) when the signal is of one polarity and reduces to form silver and chloride ions when of the opposite polarity (with Ag.sup.+ and Cl.sup.- ions being formed from the silver chloride and an electron being added to the Ag.sup.+ ion to form silver once again). The net result after each cycle of current is that the electrode remains substantially unchanged. When a DC signal is applied however, a continual reduction or continual oxidation reaction will occur. When using the electrodes to detect biological or physiological signals these reactions are practically insignificant since high impedance instruments are used to detect these signals so as to draw very small currents. As such the reactions are extremely slow. In the situation, however, where large DC currents (such as those currents generated by the Zimmer stimulator) are transmitted through an electrode used as an anode pad, a continual oxidation reaction of the silver would occur so as to form silver chloride. As silver chloride is formed the impedance at the interface between the gel and the silver chloride increases to the point where the electrode no longer is useful since a current drop through the electrode will occur.
Although the silver coated stud members of the previously described electrodes have been made with a precise amount of silver, it has been found that these electrodes function inadequately as anode pads. The surface area of the stud member that interfaces with the gel is relatively small such that the useful life of the electrode becomes somewhat unpredictable Further, while in use, over prolonged periods of use pressure on the stud member against the skin tends to force the gel away from the stud member so as to jeopardize the low impedance current path between the silver of the stud member and the gel, thereby increasing the impedance of that electrical path.